1. Field of the Invention
The present invention relates to a pre-plasticization type injection molding machine with a plasticizing part provided separately from an injection part, and more particularly, to the construction of an injection nozzle and injection chamber edge of a molding machine for eliminating the possibility of material retention in the injection chamber.
2. Description of the Prior Art
The pre-plasticization type injection molding machine comprises, in general, a plasticizing part having a screw, and an injection part having an injection plunger. Dissolved resin plasticized in the plasticizing part is supplied to the injection part's chamber by way of a conduit and is injected in the mold cavity through a nozzle from the injection plunger. Conventionally, there is provided a ball check valve in the conduit to prevent resin in the injection chamber from flowing backward to the plasticizing part when the injection plunger injects the resin.
As the present pre-plasticization type injection molding machine is equipped with an independent plasticizing part, it is more powerful in plasticizing than an in-line type injection molding machine, and works especially effectively in high-cycle molding. The ball check valve readily causes, however, dissolved resin retention, and such resin is prone to thermal decomposition and may cause trouble.
Lately the applicant proposed a pre-plasticization type injection molding machine without a ball check valve to resolve the above-stated problem. As shown in FIG. 9, an injection unit U' of a pre-plasticization type injection molding machine is composed of a plasticizing part 1 and an injection part 2 which are separately provided. The plasticizing part 1 has a plasticizing chamber 15 consisting of a heating cylinder 11, and the chamber is fitted with a screw 12. The cylinder 11 is provided with a driving part 4 at the rear. The driving part 4 has a built-in revolving drive unit and also a built-in linear drive unit to rotate and move the screw 12 forward.
The heating cylinder 11 is provided with a hopper (hopper port only is designated by 13). Resin material supplied from the hopper to the plasticizing chamber 15 is pressurized as it advances when the screw 12 rotates. It will be dissolved due to compression heat generated when kneaded by means of the screw 12 and heated by a heater 16 suitably arranged on the cylinder 11. Dissolved resin 14 is pushed forward toward a conduit 10 built at the tip of the cylinder 11.
The injection part 2 comprises an injection cylinder 20 having an injection nozzle 3 at its head. A plunger 21 is fitted in an injection chamber 22 formed by the injection cylinder 20. A driving part 5 is connected integrally with the rear end of the plunger 21. Upon instruction from a control part 50, an electromagnetic valve 51 is opened to supply a hydraulic chamber 53 of the driving part 5 with hydraulic oil to enable the plunger 21 to advance (moving to left in FIG. 9) to push dissolved resin in the injection chamber 2 into a mold cavity (not shown).
There is a conduit 30 ahead of the injection chamber 22, and the conduit 30 is connected with the conduit 10 in the plasticizing cylinder 11 to allow dissolved resin material 14 to flow from the plasticizing chamber 15 to the injection chamber 22.
In plasticizing and weighing, as the screw 12 starts to turn, it retreats to produce a conduit opening between the screw 12 tip and the conduit 10. With this, dissolved resin is fed to the injection chamber 22 of the injection part 2. In injecting, on the other hand, the screw 12 advances without rotating to close the conduit 10 with the tip, and dissolved resin is thereby stopped from flowing backward to the plasticizing chamber 15.
In particular, a pre-plasticizing type injection molding machine which moves the screw forward to prevent back from the injection chamber is able to eliminate dissolved material retention where a check ball valve is arranged. In addition, it provides not only exact weighing and very steady plasticization, but also high performance in reproducing molding conditions and rise characteristics and stability. It is also to be noted that the injection molding machine of the type realizes CAI (computer aided injection), computerizing a mold manufacturing machine with an injection molding machine on the basis of CAD (computer aided design) and CAM (computer aided manufacturing).
The injection molding machine U' has, in the meantime, an injection orifice 31 open (31a) to the center of a nozzle side-wall 34 in the injection chamber 22 and the plasticizing part conduit 30 is open (30a) to the periphery of the wall 34 of the cylinder as shown in FIG. 10.
For the purpose of maintaining pressure after injection, there is a clearance of several millimeters between the nozzle wall 34 and the injection plunger 21 at the advance position, which provides the resin with a specified holding pressure so as not to let sink marks be produced in the mold because of cooled resin in the mold cavity. Upon holding with pressure, weighing starts again, and dissolved resin flows with force from the conduit opening 30a to the injection chamber 22. Resin flow from the conduit opening 30a pressurizes the plunger head 21a since the plunger front 24' is made flat. Therefore, the new resin flow is not introduced to a closed space A opposite to the nozzle orifice opening 31a of the opening 30a, which makes the injection plunger 21 retreat. This is because resin flows from the conduit opening 30a in the arrow c direction as shown in FIG. 11. Accordingly, resin not washed in the new resin flow is retained in the closed space A, forming a deposit of resin.
As the plunger 21 retreats, dissolved resin flowing into the injection chamber 22 moves axially toward the injection cylinder 20 as shown in FIG. 12. Dissolved resin flows at the time as if it washes the injection cylinder 20 interior. Even then, there is retained resin in the space A opposite to the injection chamber 22 conduit opening 30a as the conduit opening 30a is open to the nozzle side-wall 34 at a biased position.
In addition, the injection orifice 31 is in the center, no resin flows, when injected, circumferentially toward the injection chamber 22, and almost no flow is caused with lets resin come off the injection chamber nozzle side-wall 34. Therefore resin in the closed space A opposite to the conduit opening 30a is pushed against the nozzle side-wall 34 and is readily retained without being injected when the plunger advances to the limit, though resin is somewhat affected by the advance speed of the plunger 21.
In particular, dissolved resin coming into the injection chamber from the conduit opening 30a flows in an arc along the injection cylinder 20 face due to its viscosity (streamline d) or flows as if crossing the injection chamber tip diametrically and reversely along the cylinder face (streamline e) as shown in FIG. 13. These flows d and e into the injection chamber form a meeting region A.sub.1 and a distributary region A.sub.2 where resin readily remains behind.
It is necessary to completely purge the remaining resin in the injection chamber 22 to change resin material colors (including the materials themselves). If there is a portion A in the injection chamber 22 in which it is difficult to wash away dissolved resin, even if only a little, purging must be carried out more often, and a quick changing of the arrangements may be prevented. If a material which is very thermally sensitive is put into use, the remaining resin may suffer adverse changes in chemical structure even when continuous injection molding is performed. Furthermore, permanent changes in physical properties may happen, deteriorating (aging) the quality of the resin.
Such a problem will be further revealed when a pre-plasticization injection molding machine, designed to prevent resin from flowing by advancing and retreating a screw, is operated to carry out more efficient and more accurate injection molding.